The generation of electricity by advanced gasification combined cycle power generation systems offers the potential for reduced power cost and lower environmental impact than standard coal-fired power plants. In these advanced systems, coal or other carbonaceous material is gasified with oxygen and the produced gas is cleaned to yield a low-sulfur fuel gas. This fuel gas is utilized in a combustion turbine generation system to produce electric power with reduced environmental emissions. The growing interest in gasification combined cycle (GCC) technology in recent years has been stimulated by the higher efficiency and demonstrated reliability of advanced combustion turbines, coal gasification processes, and air separation systems which are utilized in integrated gasification combined cycle (IGCC) systems. The proper integration of these three main components of an IGCC system is essential to achieve maximum operating efficiency and minimum power cost.
A general review of the current art in GCC and IGCC power generation systems is given by D. M. Todd in an article entitled "Clean Coal Technologies for Combustion turbines" presented at the GE Turbine State-of-the-Art Technology Seminar, July 1993, pp. 1-18. A. K. Anand et al present a review of the factors involved in the design of IGCC systems in a paper entitled "New Technology Trends for Improved IGCC System Performance" presented at the International Combustion turbine and Aeroengine Congress and Exposition, Houston, Tex., Jun. 5-8, 1995. A review of various integration techniques and the impact thereof on GCC economics is given in a paper by A. D. Rao et al entitled "Integration of Texaco TQ Gasification with Elevated Pressure ASU" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994.
In a paper entitled "Improved IGCC Power Output and Economics Incorporating a Supplementary Combustion Turbine" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994, A. R. Smith et al review several modes of integration between the combustion turbine and the air separation system in an IGCC process. In one mode, the air separation system feed air is provided by a separate compressor and a portion of the nitrogen product from the air separation system is compressed and introduced into the combustion turbine combustor. This nitrogen-integrated mode allows operation of the IGCC system at increased combustion turbine power output and reduced NO.sub.x formation. In an alternative operating mode, nitrogen integration is combined with air integration in which a portion of the feed air for the air separation system is supplied by extracted air from the combustion turbine compressor. This alternative mode, defined as air and nitrogen integration, gives greater operating flexibility and allows for a higher degree of optimization during IGCC system operation at part load and other off-design conditions.
Air- and nitrogen-integrated combustion turbine/air separation systems are described by representative U.S. Pat. Nos. 3,731,495, 4,019,314, 4,224,045, 4,557,735, 4,697,415, 5,081,845, 5,386,686, 5,406,786, and 5,410,869, and UK patent Application 2 067 668 A.
Combustion-based power generation systems, including IGCC systems, are subject to periods of operation below system design capacity due to changes in ambient air temperature and/or the cyclic demand for electric power. During these periods, such systems may operate below maximum design efficiency. The equipment selection and process design of an IGCC system therefore must address steady-state operation at design capacity as well as operation at off-design, part load, or turndown conditions. The air- and nitrogen-integrated IGCC system described above is a preferred option because of the potential for operating such a system at maximum overall efficiency, particularly when the system also must operate at off-design conditions.
The invention disclosed below and defined by the claims which follow addresses the need for improved methods to operate advanced power generation systems, and in particular describes the improved operation of air- and nitrogen-integrated combustion turbine and air separation systems at off-design conditions.